U.S. Pat. No. 4,514,771 discloses a rotating memory disk with a surface divided into a number of concentric track zones each containing a predetermined number of tracks. An apparatus varies the rotational speed of the disk in accordance with the radial position of a transducer relative to the disk. The rotational speed of the disk is maintained constant while the transducer is within a given zone. The apparatus includes a controller which monitors the actual motor speed by way of a tachometer feedback signal, and which effects a change in the actual motor speed if it varies from the desired motor speed.
U.S. Pat. No. 4,530,018 discloses a data recording/reproducing head and an optical disk having a plurality of data-storing tracks organized as track groups. The optical disk is rotated by a motor. As the head is tracing tracks included in the same track group, the disk is made to rotate at a constant angular speed. When the head tracing is changed among the track groups, a motor control section changes the angular velocity of the disk to a desired velocity. To accomplish this, the motor control section phase-compares a rotation signal from the motor with a reference oscillation signal and produces a rotation control signal which controls the motor.
U.S. Pat. No. 5,315,571 discloses an information recording and reproducing device for use with compact disks (CDs) and CD-ROMs (Compact Disc-Read Only Memory). The device writes the reproduced data into a memory means by using a writing clock in synchronization with the reproduced data from the recording medium. The reproduced data is then read out from the memory means by using a first reading clock in synchronization with the reproduced data. In another embodiment for use with information, such as music, which must be reproduced at a predetermined rate, a switching means selects a second reading clock having the determined reference frequency as a reading clock.